Converting Amps to Kilowatts

SensorNote 120: Operation notes for safety and productivity

Revenue protection professionals use the input of various sources to point them toward locations where there is suspicion of power diversion. When out in the field to investigate, utility personnel should determine if there is a discrepancy from the source to the revenue meter. Because the revenue meter displays the load in either Watts or Kilowatts, the Amp reading taken at the source will need to be converted. This conversion allows for a quick field verification to identify any inefficient loading.

How to calculate Wattage: Amps x Volts = Watts*
How to calculate Kilowatts: (Amps x Volts)/1000 = Kilowatts

First, take a current reading at the service entrance. On an overhead service, a quick amp measurement can be taken with a live-line ammeter on the two power leads going to the customer’s weather head.  Or, for an underground service, a quick measurement taken at the pad mount transformer.

The nominal voltage for a single phase service in North America is 240 VAC Line to Line or 120 VAC Line to Neutral. The ANSI C84.1 standard states voltage is to be within +/- 5% . With this understanding, it is acceptable to assume the measurement by using the nominal voltage if it is not possible to take a direct voltage reading with a live-line Voltmeter.

To provide an example, side A of the feed is 28 Amps, and side B of the feed is 32 Amps. Knowing the Voltage is approximately 120V, the Kilowatts can be quickly calculated as follows:

(28 + 32) x 120 = 7200 Watts
7200/1000 = 7.2 Kilowatts

With the calculated Kilowatts, you can now review the readings on the revenue meter for comparison.

Compare the Source measurement to a Solid State Meter

Note the reading on the Solid State Meter, either remotely or from a hand held device, and compare this measurement to the Watts or Kilowatts calculated from the source.

Compare the Source measurement to an Electromechanical Meter

The Kilowatt Hour (Kh), disk revolutions, and time in seconds are needed to calculate the Watts on an Electromechanical Meter. Use the Kh noted on the nameplate of the meter. To determine the load for a specific time, count the number of times the disk rotates during an interval of at least 60 seconds.**  

How to Calculate Load (in Watts): (Kh x disk revolutions x 3600) / (time in seconds)
Example: A meter with a Kh of 7.2 made 14 revolutions in 70 seconds

Load (in Watts): (7.2 x 14 x 3600) / (70) = 5148 Watts
Load (in Kilowatts): (5148) / (1000) = 5.148 Kilowatts


It is quick and easy to compare source and revenue meter measurements when they are in the same metric. Further investigation to determine where the diversion has occurred is necessary when the difference between the source and the meter is larger than the utility allows for with technical losses.

*Assumption is made that Power Factor is at unity
** A Pocket Guide to Watthour Meters 3rd Edition February 2006 - Alexander Publications